The present invention relates to a process for the prepartion of ethylbenzene by alkylation of benzene with dilute ethylene contained in refinery dry gas. More particularly, the invention relates to a process for the preparation of ethylbenzene by alkylation of benzene with dilute ethylene contained in dry gas by catalytic distillation, also to an apparatus used for the catalytic distillation process.
Ethylbenzene is an important organic chemical material which is mainly used to produce styrene. The vigorous market need for styrene brings along the continued increase of ethylbenzene production. Therefore, it is of great importance to find out new material sources and cheaper route for ethylbenzene preparation.
As the main raw material in production of styrene monomer, almost 90% of ethylbenzene are produced by the alkylation of benzene with pure ethylene. The ethylene source is very important for the development of ethylbenzene industry, which has a bearing on the economic effectiveness of ethylbenzene industry and on smoothing operation. Making full use of various ethylene sources, especially dilute ethylene, will certainly expand the output of ethylbenzene production.
Catalytic cracking is an important process for deep transformation of heavy oil in refineries which produces dry gas containing a dilute ethylene in an amount of about 10-20% by mole. Most of dry gas are used as fuel for furnaces in past time, which represents a great waste of useful resource. The use of ethylene in dry gas to produce ethylbenzene will extend the of ethylene source, open up a new way of comprehensive utilization of dry gas and obviously increase economic effectiveness.
In recent decades, the use of dilute ethylene for ethylbenzene production has drawn a widespread attention in the world and has achieved rapid progress. The common processes for ethylbenzene production by the alkylation of benzene with ethylene fall roughly into two categories, one is gas-phase method, and the other is liquid-phase method. In gas-phase method both ethylene and benzene are kept in gas phase and the alkylation to ethylbenzene is proceeded over a solid catalyst, which belongs to gas-solid reaction type and has a long history of industrial application. The recent representative process of gas phase method is Mobil/Badger technology, which was developed by Mobil Oil Company and Badger Engineering Company in 1970s and got continued improvement later. Hydrocarbon Processing (Part of Petrochemical Processes""95, March 1995, page 115) reported the third generation of process for the ethylbenzene preparation by gas-solid phase alkylation of benzene with dilute ethylene contained in dry gas which took over ZSM-5 zeolite. EP-0308099 disclosed a process for the preparation of ethylbenzene by gas-phase alkylation over ZSM-5 zeolite, and a process for the transalkylation of benzene with diethylbenzene to ethylbenzene over ZSM-5 and ZSM-11 zeolites. The main advantages of the gas-phase method are as follows: ethylene stock needs not to be in a high purity, it is suitable for various ethylene sources and various content of ethylene in the range of 10-100% by mole, high catalyst capacity, no corrosion and pollution, simple technological process with high utilization of energy. But the gas phase process also has obvious disadvantages: rigorous control of the concentration of harmful impurity such as H2S, O2, CO2, H2O etc. in dry gas below 1 ppm, high reaction temperature and middle pressure, for example, the alkylation needs to be operated at 300-500xc2x0 C. and 0.1-20.8 MPa, as described in the above patents, which leads to high energy consumption, strict requirements for the apparatus and for the operation. Moreover, the reaction produces more by-products, the ethylbenzene selectivity is not high, ZSM-5 zeolite is more likely to deactivate by coking, the concentration of dimethylbenzene in product ethylbenzene amounts to 1000-2000 ppm (industry required below 100 ppm) which negatively affects the quality of polystyrene.
In addition, CN 1031072A (applicant: No. 2 petroleum works, Fushun Petrochemical Company, China) and CN 1074392A (applicant: Dalian chemical physics institute, Chinese Science Academy) also reported the modified commercial ZSM-5 catalyst and its preparation. When used for gas-phase alkylation to produce ethylbenzene, the ZSM-5 catalyst is likely to deactivate by coking due to high reaction temperature, and the concentration of dimethylbenzene in the product mixture is high, which make the disadvantages of the gas phase method mentioned above unavoidable.
Another kind of process for the alkylation of benzene with ethylene to produce ethylbenzene is called liquid-phase method, in which both ethylene and benzene are kept in liquid phase under high pressure and the alkylation reaction takes place over a solid catalyst. The liquid phase method belongs to liquid-solid reaction type.
The representative process of liquid-phase method is Unocal/Lummus/UOP technology which produces ethylbenzene by liquid-phase alkylation of benzene with ethylene over a solid acid catalyst containing Y zeolite. Compared with gas-phase method, the liquid-phase method has the following features: the reaction takes place in liquid phase, the temperature is generally below 300xc2x0 C., the lifetime of catalyst is long, the selectivity of ethylbenzene is high, the concentration of dimethylbenzene in product ethylbenzene is just below 100 ppm, which fully satisfies the requirement for high quality styrene. Hereafter, many publications reported the improvement of the process for liquid-phase alkylation for ethylbenzene production and the catalyst and its preparation. For instance:
G. Bellussi et al (Journal of Catalysis, 1995, 157, 227-234) disclosed that xcex2 zeolite had higher activity and selectivity than that of Y zeolite in the liquid-phase alkylation of benzene, and proposed a mechanism wherein the activity of xcex2 zeolite was controlled by internal diffusion within particles. This reference only discussed general questions of liquid-phase benzene alkylation with lower olefin, and compared the activities and selectivities of various zeolites of different types.
Hydrocarbon Processing (Part of Petrochemical Processes""95, March 1995, page 114) reported Unocal/Lummus/UOP process in which fixed bed was used to produce ethylbenzene by liquid phase alkylation over a zeolite, but it didn""t relate to the specific catalyst and operation conditions.
U.S. Pat. No. 5,145,817 (Dow Chemical Company) described a bimetal ultrastable Y-type zeolite used in the liquid-phase alkylation and transalkylation of benzene to produce ethylbenzene, in which the pressure of the reactor was 35 atm and the temperature was 223-301xc2x0 C.
U.S. Pat. No. 5,030,786 (Fina Technology Company) relates to a process for alkylation/transalkylation of aromatics to produce ethylbenzene, in which dehydrated aromatics was fed into the reaction zone containing zeolite catalyst to carry out a liquid reaction over one of Y, xcexa9 and xcex2 zeolites at a temperature of 225xc2x0 C. and a pressure of 24.5 atm.
CN 1096470A described a kind of xcex2-zeolite/xcex3-alumina catalyst used for liquid-phase benzene alkylation, in which 0.5-10% by weight, preferred 1-5% by weight fluorine or chlorine was incorporated so as to improve the activity of catalyst. In its example 4, the condition of 200xc2x0 C. and 2.94 MPa was used to evaluate catalyst.
Although liquid phase method takes outstanding advantage over gas phase one, it also has obvious weakness. The reaction pressure of liquid phase method is far greater than that of gas-phase method, which leads to increase of energy consumption, complexity in apparatus and in operation. The more unfavorable is that the liquid-phase method requires pure ethylene as feed and dilute ethylene is not usable, so liquid phase method can not be adopted to produce ethylbenzene using dilute ethylene stock.
In addition to the gas phase method and liquid-phase method, CN 1207960A (applicant: No.2 Chemical Works, Beijing Yanshan Petrochemical Corporation and Beijing Institute of Clothing Technology) reported a new process for gas-liquid phase alkylation of benzene with dilute ethylene contained in refinery dry gas to produce ethylbenzene over a new xcex2 zeolite catalyst, in which benzene was fed in liquid-phase while dry gas was fed in gas phase, two feeds countercurrently flowed through the reactor and reacted over the catalyst in the fixed bed, the product mixture was drawn from the reactor and was separated to obtain ethylbenzene. Belonging to gas-liquid-solid three phase reaction type, the process combines the main advantages of the gas-phase method and the liquid-phase method. The temperature and pressure in gas-liquid-solid process are both low, the requirements for apparatus and operation is not strict, both the conversion of ethylene and selectivity of ethylbenzene are high, the concentration of dimethylbenzene in product is very low, and the lifetime of catalyst is long. The more important is that the gas-liquid-solid process may use dilute ethylene as feed. However it seems that the process needs improvement, due to high external reflux ratio which leads to heavy burden for separating benzene and ethylbenzene, high molar ratio of benzene to ethylene in feed which limits the catalyst capacity, and the selectivity of the product ethylbenzene also needs to be enhanced, etc.
U.S. Pat. No. 5,476,978 disclosed a process for the alkylation of benzene with ethylene to produce ethylbenzene by catalytic distillation, in which ethylene and benzene in excessive amount were respectively fed into the catalytic distillation tower to take alkylation reaction and distillation separation simultaneously. The process takes advantages of catalytic distillation, and the extent of catalyst utilization and product quality is high. However the process uses almost pure ethylene, so the gas flux is not large, and so liquid flooding in tower could be easily avoided. Hence there are not too much requirements for the similarity of the catalyst and the packing particles in respect of their size, structure and bulk density of the catalyst particle and the packing, neither for bed voidage. In its example, the catalyst particles were tilled in small pockets of fiber glass belts and set in a helix form with stainless steel mesh, then packed in reaction zone layer by layer. The process is not suitable for dilute ethylene feed because large amount of other gases such as N2, CO2, H2, and C1-4 paraffins will lead to a large gas flux, more likely flooding and thus to a difficult gas-liquid mass transfer. Then high bed voidage and high efficiency of mass transfer between catalyst particles and packing are required and it is difficult to satisfy in this patent.
There has been existing a need to provide an improved method for the preparation of ethylbenzene by alkylation of benzene with dilute ethylene contained in the dry gas, in which the moderate reaction conditions and simple and inexpensive operation can be used to utilize the dilute ethylene in refinery dry gas to produce ethylbenzene in high purity with high conversion of ethylene, high selectivity of ethylbenzene, high quality of product and long catalyst lifetime.
Therefore, one object of the invention is to provide a new method for the preparation of ethylbenzene by alkylation of benzene with dilute ethylene contained in refinery dry gas.
Another object of the invention is to provide an apparatus which can be used in the process for the preparation of ethylbenzene by alkylation of benzene with dilute ethylene contained in refinery dry gas.